A non-aqueous electrolyte secondary battery such as a lithium ion secondary battery is highly valued in view of high energy density. Instead of conventional secondary batteries like a lead secondary battery, a nickel-cadmium (Ni—Cd) secondary battery, a nickel-hydride secondary battery, the non-aqueous electrolyte secondary battery is becoming the major player of the compact size secondary battery for electronic devices. This secondary battery is further expected to be used for power system of automobiles and so on. In order to obtain higher power density, various studies were made over the issues like: pursuing higher capacity of active materials and higher filling of the same in the electrode; thickening of electrode layer; and making the separator and the current collector be thinner.
However, when blending ratio of binder is reduced to densely-fill the active materials in the electrodes, the electrodes become brittle, which tends to cause an increase of fraction defective in the cutting process thereof and assembly process of the battery, but also cause a decline of batteries' cycle characteristic. In addition, when blending ratio of an electroconductive material is reduced in the electrodes, there is a problem that rate performance of the battery declines.
Moreover, if the surface of electrodes is not flat and smooth, the distance between the positive and the negative electrodes separated by a separator becomes locally closer. Thereby electrical current concentrates on a particular point and there is a fear of cause of local heat. So, in general, by using methods like roll-press after forming of electrode layers, the surface of electrodes is made flat and smooth, and density of electrode layers can be improved. Nevertheless, if the electrode layers before roll-press have rough surface and low density, the smooth surface is hard to be obtained even by a treatment with the larger compressed pressure. If filling ratio of active materials is forcibly raised with high pressure, microscopic cracks grow within the electrode layers at a time of compression. As a result, the electrode layers become brittle, which cause an increase of defective in the cutting process thereof and the assembly process of the battery.
When thicken electrode layers, it is capable to increase the ratio of active materials in the battery, therefore it is advantageous for enlarge the energy density. However, the thickened electrode layers have problems like: an increase of defective in the assembly process of battery due to the lack of flexibility of the electrodes; and a decline of battery's rate performance as a result of the longer average distance to the current collector.
An idea adopting rubber-type materials as a binder is proposed to enhance the flexibility of electrodes and to maintain the above-mentioned cycle characteristic in order to reduce the damage of electrodes in the cutting and winding processes (e.g., Patent Documents 1˜3). Another idea is proposed to obtain an electrode which is excellent in flexibility and adhesiveness by using reactive-hardening type binder (Patent Document 4).
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 5-74461
Patent Document 2: JP-A No. 2000-200608
Patent Document 3: JP-A No. 2000-228197
Patent Document 4: JP-A No. 6-325752
Patent Document 5: U.S. Pat. No. 5,846,674